The present invention relates to gradient index optical fibers and, more particularly to a bundled array of such fibers forming a lens array which transmits an image of an object at an object plane to an image plane at a magnification other than unity.
Image transmitters comprising bundled gradient index optical fibers are known in the art. U.S. Pat. No. 3,658,407 describes a light conducting fiber made of glass or synthetic resin which has a refractive index distribution in a cross section thereof that varies parabolically outward from a center portion thereof. Each fiber acts as a focusing lens to transmit part of an image of an object placed near one end. An assembly of fibers, in a staggered two-row array, transmit and focus an image of the object. The fiber lenses are produced under the trade name "SELFOC"; the mark is registered in Japan and owned by Nippon Sheet Glass Co., Ltd.
Numerous techniques are known in the art for manufacturing glass or plastic fibers with index-of-refraction variations. These are usefully summarized in an article entitled "Gradient Index Optics: A Review" by Duncan T. Moore, Applied Optics, Apr. 1, 1980, Volume 19, No. 7, pp. 1035-1038.
Gradient index lens arrays have found use in a number of technologies; e.g. in construction of printed type optical circuits as disclosed in U.S. Pat. No. 3,922,062 and as a replacement for conventional optical systems in copiers as disclosed in U.S. Pat. Nos. 3,947,106 and 3,977,777.
The lens arrays described above can be described as unity magnification lens arrays in that they transmit images from an object plane to an image plane at a magnification ratio of 1:1. In a copending U.S. application Ser. No. 151,994 filed on May 21, 1980, now U.S. Pat. No. 4,331,380, there is described a gradient index lens array which transmits reduced or enlarged images, the specific magnification depending upon the geometry and orientation of the fibers comprising the array. Unlike the unity magnification lens arrays of the prior art, which produce a uniform exposure level at an imaging plane, the reduction array creates an exposure level which is minimum in the array center and increases outward towards the ends of the array. Since image exposure is desired to be uniform at all points, some compensation must be included in the system geometry. A further characteristic of the reduction array is the deterioration of image quality towards the ends of the array. It would be most desirable to simultaneously compensate for both exposure and the image quality. This has been accomplished, according to the present invention, by analyzing the system geometry and, particularly, the effects of selectively altering the fiber aperture in the array direction and then introducing appropriate modifications into the lens array.